Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3116394.

This is a loop for a Swatch watch strap, but it's quite generic and would probably fit other watches. There are two versions: loop1 has internal dimensions of 17x3.5mm and 1.2mm wall thickness, and loop2 has internal dimensions of 17x3.4mm and 0.8mm wall thickness. In my case, I used the second version with thinner 0.8mm walls.

To fit the loop, I used a metal pin to push out the strap retaining pin, removed the strap, fitted the loop then reassembled.